Extrusion-type coating head for coating a magnetic recording medium

ABSTRACT

A coating method and an extrusion-type coating head in which turbulence at the junction between two flows of coating materials is prevented to provide multi-layer simultaneous coating over a wide range of coating rates and with a good coating quality. An intermediate block is sandwiched betwen a rear block defining a back edge and a front block defining a doctor edge. Two coating flows pass through slits formed on either side of the intermediate block, joining at the top end of the intermediate block and flowing together into a short slot extending between the end of the intermediate block and the outlet of the head.

This is a divisional of application Ser. No. 07/376,268, filed July 3,1989, now U.S. Pat. No. 5,030,484.

BACKGROUND OF THE INVENTION

The present invention relates to a method for simultaneously applying aplurality of coating solutions onto a moving support, thereby to formmultiple layers of coating solutions thereon. More particularly, theinvention pertains to a method for manufacturing a multi-layer magneticrecording medium by simultaneous coating.

Recently, magnetic recording media have been improved in recordingdensity and decreased in thickness. In association with this technicaladvancement, the current tendency is to use a multi-layer magneticrecording medium instead of a single-layer magnetic recording medium.

In order to improve the adhesion of a magnetic layer or back layer andits support, an undercoating layer is provided on the support. That is,in order to manufacture magnetic recording media, it is necessary toform two or more layers on one support.

In manufacturing such a multi-layer magnetic recording medium, coatingand drying are carried out after each layer is formed on the support.That is, coating and drying operations must be repeatedly carried out tomanufacture a single multi-layer magnetic recording medium. Such amethod though is low in productivity and high in the cost of requiredequipment.

Hence, there has been a strong demand for the provision of a method,hereinafter referred to as "a multi-layer coating method", for forming aplurality of layers on a support in one coating and drying cycle.

Examples of known magnetic recording medium coating methods are rollcoating, gravure coating and extrusion-plus-doctor coating methods. Inthese coating methods, however, it is impossible to simultaneously forma plurality of film layers on one support because of the shear stresswhich is produced during the coating operation.

On the other hand, a slide coating method using a slot has beendisclosed, as one example of a multi-layer coating method, for themanufacture of photographing photosensitive materials. JapanesePublished Patent Application No. 12937/1981 discloses a multi-layercoating method for magnetic recording materials which uses a coatingapparatus with a slide. However, in the manufacture of magneticrecording media, the coating solutions have a high viscosity and dryquickly being organic solvent dispersion solution which are liable toaggregate, and are hence this method is not suitable for high-speed filmlayer coating operations.

In addition, a variety of extrusion-type coating methods have beendisclosed in the art; however, almost all are limited to forming asingle film layer on a support.

Japanese Unexamined Published Patent Application (OPI) No. 109162/1983discloses a coating method in which an extrusion coating head is pressedagainst a web (support) is employed to form a plurality of film layersthereon. However, the method is advantageous in the following points:(1) It is necessary to provide a doctor edge for each layer. The edgesare liable to wear, and it is rather difficult to set the edges at theproper position. (2) The layers are formed separately, and therefore thestable coating range of each layer is small. Particularly the stablecoating range of the lowermost layer (the layer directly above thesupport) is small.

On the other hand, Japanese Published Patent Application No 8977/1958and Japanese Patent Application (OPI) No. 111168/1986 disclose a methodin which a plurality of coating solutions are allowed to meet togetherin a hopper (pocket) or slot inlet and are then extruded. These methodsare still disadvantageous in the following points: (1) In the case wherethe slot inlet is divided with a partition board or the like, it istechnically difficult to make the gap between the partition board andthe slot uniform in the widthwise direction. Accordingly, it isimpossible to make the coating thickness uniform in the widthwisedirection. (2) When coating solutions differing in viscosity are allowedto flow over a long distance after meeting, the solutions may be mixedin the interface therebetween. Thus, the flow of the solutions becomesunstable.

U.S. Pat. No. 2,901,700 discloses a coating method in which anintermediate edge having a sharp or flat end face is arranged in theslot to apply two coating solutions to the support. In this case, sincethe intermediate edge is made of an ordinary material, small flawsseveral to several tens of microns in length are inevitably formed onthe end face. As a result, strips are formed on the product. In the casewhere an intermediate edge having the flat end face is used, dependingon the width of the flat end surface, the width of the solution path maychange abruptly, as a result of which vortices are formed in the streamof coating solutions, whereby the two solutions are mixed, spoiling theinterface therebetween. Hence, the resultant product also has strips onits surface.

The above-described difficulties become more significant as the coatingspeed increases and the coating thickness is reduced.

SUMMARY OF THE INVENTION

Accordingly, an object of the present invention is to eliminate theabove-described difficulties accompanying a conventional multi-layercoating method. More specifically, an object of the invention is toprovide a multi-layer coating method in which a plurality of film layerscan be applied to the support at high speed and the resultant product isuniform in surface quality, having no defects such as stripes, and theinterface of the coating solutions is maintained uniform.

The foregoing and other objects of the invention have been achieved bythe provision of a coating method in which, with an extrusion-type headhaving a back edge and a doctor edge pressing against a support which isbeing conveyed, coating solutions at least one of which is a magneticsolution are simultaneously applied onto the support to form at leasttwo layers of coating solution thereon, in which, according to theinvention, the coating solutions meet at an intermediate block arrangedin the slot of the head near the outlet of the slot in such a manner asto prevent the coating solutions from being mixed at the interfacethereof.

In the coating method of the invention, a plurality of coating solutionsof different viscosities meet in the slot, thus forming multiple layersof coating solutions. However, in order to eliminate the difficulty thatthe interface of the coating solutions is made irregular by the flowingof the multiple layers over a long distance in the slot, the solutionmeeting position is suitably selected, as a result of which the coatingsolution layers are not mixed. In addition, the hardness andconfiguration of the intermediate block are suitably selected in orderto perform a simultaneous multi-layer coating operation with highaccuracy.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram showing an example of an extrusion-type headaccording to the invention;

FIG. 2 is an enlarged view of the end portion of the extrusion-type headillustrated in FIG. 1;

FIGS. 3, 4 and 5 are sectional views showing different examples of anintermediate block used in the extrusion-type head according to theinvention; and

FIGS. 6A and 6B are sectional views showing other examples ofextrusion-type heads according to the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Preferred embodiments of the present invention will be described withreference to the accompanying drawings. FIG. 1 is a diagram showing incross section an extrusion-type head constructed in accordance with afirst preferred embodiment of the invention.

In FIG. 1, a reference numeral 1 designates a rear block defining a backedge; 3, a front block having a doctor edge; and 2, an intermediateblock. At least the front end of the intermediate block 2 is made of amaterial having a Rockwell scale A hardness greater than 80, forexample, carbon steel, nitride steel, cement steel, cemental carbide, orceramic. Magnetic solutions of different viscosities, namely, coatingsolutions 6 and 7, are supplied into pockets 4 and 5 by respective pumps(not shown), and are passed through narrow slits, meeting in the slotformed at the ends of the slits. The coating solutions are then appliedonto a support (web) 8 which is conveyed in the indicated direction.

The extrusion-type head will be further described with reference to FIG.2.

FIG. 2 is an enlarged view showing the region where the two coatingsolutions meet. In FIG. 2, reference character Z indicates the end ofthe intermediate block 2; X, the top of the back edge; and Y, the top ofthe doctor edge. The end Z must be located a distance B below (in FIG.2) the line which passes through the tops X and Y. The distance B shouldbe as small as possible. If the two coating solutions of differentviscosities after meeting are caused to flow in the form of amulti-layer over a long distance in the slot while under pressure, theinterface of the two coating solutions will become irregular, as aresult of which the two solutions may be mixed at the interface.However, if the solution meeting point is set extremely close to theoutlet of the slot, such mixing is satisfactorily prevented. Thedistance B should be in a range of from 0.01 mm to 20 mm, preferably0.01 to 5 mm, more preferably 0.01 to 1 mm. The distance B should bedetermined according to the viscosity and flow rates of the two coatingsolutions.

Further in FIG. 2, reference character C designates the width of theslit in which the coating solution forming the upper layer flows; C',the width of the slit in which the coating solution forming the lowerlayer flows; and D, the width of the slot in which flows the multi-layerformed by the upper and lower layers after they have met. The widths C,C' and D should be in a range of from 0.03 mm to 3 mm depending on theviscosity and flow rates of the coating solutions. In this connection,it is preferable that the widths C and C' be so selected that the ratioof the flow rates in the respective slits is in a range of 1:1 to 5:1 inorder to prevent turbulence at the interface where the two coatingsolutions meet.

FIGS. 3, 4 and 5 are enlarged sectional views showing examples of theend portion of the intermediate block 2.

In the invention, at least the end portion 2a of the intermediate block2 is made of a material having a Rockwell scale A hardness of 80 orhigher. In the case where the end portion 2a is flat as shown in FIG. 3,the width of the flat surface is less than 1 mm, preferably less than0.5 mm. In the case where the end portion 2a is sharply tapered as shownin FIG. 4, the taper angle θ is more than 25°, preferably more than 50°.In the case where the end portion 2a is round as shown in FIG. 5, theradius R is less than 1 mm, preferably less than 0.5 mm.

The end portion 2a of the intermediate block 2, by having one of theconfigurations described above, eliminates the difficulty of the twocoating solutions being made turbulent at the point where they meet toform, for instance, vortices. In other words, the end portion thusformed allows the two coating solutions to smoothly meet, with theresult that the upper and lower layers are not mixed and the resultantmulti-layer is satisfactory.

In a coating apparatus with the extrusion type head thus constructed,the above-described support 8 (FIG. 1) is laid over a conveyance guidestructure such as guide rollers 9 under a substantially constant tensionin such a manner that the support 8 curves slightly in the direction ofits thickness. The support 8 is thus set close to the head with the aidof an extruder support mechanism (not shown) in such a manner that itcurves substantially parallel to the downstream edge surface of thedoctor edge. Under this condition, a solution supplying system isoperated to supply the coating solutions 6 and 7 at predetermined flowrates. The coating solutions 6 and 7 thus supplied are extruded to theoutlet of the slot through the pockets 4 and 5 and the slits with auniform flow rate and pressure distribution over the width of thesupport 8.

As described above, the head is slightly spaced from the surface of thesupport 8. Therefore, the extruded coating solutions 6 and 7, whileforming a bead on the edge surface of the back edge, are caused to flowalong the surface of the support, which is continuously conveyed in thedirection of the arrow A, in such a manner as to push the support 8 andthe doctor edge away from each other.

As to details of the extrusion apparatus not discussed above,conventional techniques may be used. That is, the support materials andthe dimensions of the back and doctor edges, the angles at which thesupport enters and leaves the coating head, and the configuration of thesupport can be the same as in a conventional case.

Appropriate conventional techniques are disclosed in Japanese UnexaminedPublished Patent Applications (OPIs) Nos. 84771/1982, 104666/1983,109162/1983, and 238179/1985.

The invention has the following novel effects:

In the extrusion of a multi-layer of coating solutions differing inviscosity, the provision of the intermediate block prevents adverseeffects on the flows of the coating solutions in the region where thecoating solutions meet, and it allows the solution meeting position tobe arranged extremely close to the slot outlet. Therefore, in thecoating method of the invention, various factors which could lead toinstabilities in the interface of the coating solution layers areeliminated. As a result, the high-speed film coating performance isimproved, the resultant coating is even and it has no stripes or thelike, and the stable multi-layer simultaneous coating range can beincreased, which contributes greatly to an improvement of productivity.Furthermore, in the invention, since the intermediate block is made of acemented material, breaking and burring of the end portion areessentially eliminated, and the coating conditions are improvedaccordingly.

As conducive to a full understanding of the invention, specific examplesthereof will be described.

EXAMPLE No. 1

The first layer (the coating solution 6 in FIG. 1) was a dispersionsolution containing carbon and having a composition as shown in Table 1,and the second layer was a dispersion solution of magnetic materialhaving a composition also as shown in Table 1.

The supports were made of polyethylene terephthalate (PET) webs 15 μmand 75 μm in thickness. The first and second layers were applied to thewebs with different coating rates under conditions that the coatingwidth was 400 mm, the coating speed 200 m/min, and the tension 25 kg/m(width).

In the extrusion-type head, the thickness of the back edge and thedoctor edge was 2 mm, the slit widths C and C' indicated in FIG. 1 wereboth 0.2 mm, the width D was approximately 0.4 mm, the distance B fromthe slit outlet to the end of the intermediate block was 0.1 mm, and thethickness A shown in FIG. 3 was 0.1 mm.

For comparison, the coating of a support according to the methoddisclosed in Japanese Unexamined Published Patent Application No.109161/1983 was tested.

                  TABLE 1                                                         ______________________________________                                        First layer coating solution composition:                                     ______________________________________                                        Carbon black ("Sebaruco MICT"),                                                                    200 parts by weight                                      average grain size 250 μm                                                  "Nipporan - 730" (manufactured                                                                      80 parts by weight                                      by Nippon Polyurethane Co.)                                                   Phenoxy resin PKH-1 (manu-                                                                          35 parts by weight                                      factured by Union Carbide Co.)                                                Oreic acid copper     1 part by weight                                        Methyl ethyl ketone  500 parts by weight                                      ______________________________________                                    

                  TABLE 2                                                         ______________________________________                                        Second layer coating solution composition                                     (magnetic material dispersion solution):                                      ______________________________________                                        CO containing magnetic iron oxide                                                                  100 parts by weight                                      (S.sub.BET 35 m.sup.2 /g)                                                     Nitrocellulose        10 parts by weight                                      Polyurethan resin "Nipporan 2304"                                                                   8 parts by weight                                       (manufactured by Nippon                                                       Polyurethane Co.)                                                             Polyisocyanate        8 parts by weight                                       Cr.sub.2 O.sub.3      2 parts by weight                                       Carbon black (average grain                                                                         2 parts by weight                                       diameter 20 mμ)                                                            Stearic acid          1 part by weight                                        Butyl stearate        1 part by weight                                        Methyl ethyl ketone  300 parts by weight                                      ______________________________________                                    

                  TABLE 3                                                         ______________________________________                                        First layer                                                                              Second layer coating rate                                          coating layer                                                                            cc/sq. m.                                                          cc/sq. m.  3       10    15    20  25    30  40                               ______________________________________                                         3         ◯                                                                         ◯                                                                       ◯                                                                       ◯                                                                     ◯                                                                       ◯                                                                     ◯                    10         ◯                                                                         ◯                                                                       ◯                                                                       ◯                                                                     ◯                                                                       ◯                                                                     ◯                    15         ◯                                                                         ◯                                                                       ◯                                                                       ◯                                                                     ◯                                                                       ◯                                                                     ◯                    20         ◯                                                                         ◯                                                                       ◯                                                                       ◯                                                                     ◯                                                                       ◯                                                                     ◯                    25         ◯                                                                         ◯                                                                       ◯                                                                       ◯                                                                     ◯                                                                       ◯                                                                     ◯                    30         ◯                                                                         ◯                                                                       ◯                                                                       ◯                                                                     ◯                                                                       ◯                                                                     X                                40         ◯                                                                         ◯                                                                       ◯                                                                       ◯                                                                     ◯                                                                       X   X                                ______________________________________                                         Legend:                                                                       ◯ Satisfactory multilayer coating                                 X  Unsatisfactory multilayer coating                                     

Table 3 indicates the results of coating operations in which thecombined coating rates of the first and second layers were in a range of6 to 80 cc/m². As is apparent from Table 3, the coating operation can becarried satisfactorily out when the sum of the coating rates of thefirst and second layers is in a range of 6 to 65 cc/m².

On the other hand, the method disclosed by Japanese Published UnexaminedPatent Application No. 109162/1983 was tested using coating rates of 3cc/m² and 10 cc/m² for the first layer and coating rates of 3 cc/m² to30 cc/m² for the second layer. In this test, it was impossible tosatisfactorily form the first layer, and accordingly the desiredmulti-layer was not formed.

EXAMPLE No. 2

Tests were carried out with the thickness A and curvature R of the endportion 2a of the intermediate block 2 (FIG. 5) varied.

The dimensions A and R were both set 1 mm or less, 0.5 mm, 1 mm, and 2mm. The coating rates of the first and second layers were both set to 15cc/m². A 75 μm base was employed. The value B in FIG. 2 was changedaccording to the dimension A. The other conditions were the same asthose in Example No. 1 described above.

The results of the tests were as follows: When both of the dimensions Aand R were 0.1 mm or less, the resultant layer surface was satisfactory.When both A and R were 1 mm, stripes were found on the resultant layersurface, but the product was acceptable in practical use.

On the other hand, when A and B were set to 2 mm, a number of stripeswere formed thereon, and the product was not acceptable in practicaluse. In the case where the dimensions R was set to 2 mm or more, notonly stripes but also wide irregular patterns were formed on the layersurface, and the product was not fit for practical use.

EXAMPLE No. 3

For this test, the angle of θ of the sharp end portion 2a of theintermediate block 2 shown in FIG. 4 and the hardness of the end portionwere changed. The coating conditions were the same as those in ExampleNo. 1 above. The tests results were as in the following Table 4:

                  TABLE 4                                                         ______________________________________                                        Material Rockwell    Configuration of                                         hardness A  Angle    manufactured Results of                                  (60 kg)     (θ)                                                                              end portion  coating                                     ______________________________________                                        75          50°                                                                             Many flaws and                                                                             Many stripes                                                     burrs                                                    80          50°                                                                             No flaws or burrs                                                                          No stripes                                  85          20°                                                                             Many flaws   Many stripes                                            50°                                                                             No flaws     No stripes                                              90°                                                                             No flaws     No stripes                                              120°                                                                            No flaws     No stripes                                  87          50°                                                                             No flaws     No stripes                                  90          50°                                                                             No flaws     No stripes                                  ______________________________________                                    

EXAMPLE No. 4

The composition of the second layer was as indicated in the followingTable 5, and that of the first layer was obtained by changing the HC ofCo-r-FeOx (magnetic recording material) in Table 4 to 600 Oe.

With the coating rates of the first and second layers in a range of 10to 30 cc/m², tests were carried out in the same manner as in theabove-described Example No. 1.

The results of the tests were satisfactory. That is, the surface of thecoating was satisfactory in quality.

As is apparent from the above-described example, according to theinvention, a multi-layer simultaneous coating operation can be carriedout at a wide range of coating rates, and, in the coating operation,thin film layers can be formed at higher speed, and the resultantmulti-layer is satisfactory in quality.

                  TABLE 5                                                         ______________________________________                                        Co-r-FeOx (Hc = 700 Oe)                                                                            100 parts by weight                                      Copolymer of vinyl chloride, vinyl                                                                  20 parts by weight                                      acetate and vinyl alcohol (92:3:5,                                            polymerization degree 400)                                                    Polyester polyurethane                                                                              5 parts by weight                                       (molecular weight 50,000)                                                     Oleic acid (industrial)                                                                             2 parts by weight                                       Dimethyl polysiloxane                                                                              0.2 part by weight                                       (polymerization degree 60)                                                    Carbon (grain size 10 mμ)                                                                       1.0 part by weight                                       Al.sub.2 O.sub.3 (grain size 0.5 μm)                                                            1.0 part by weight                                       Methyl ethyl ketone  250 parts by weight                                      ______________________________________                                    

EXAMPLE No. 5

Tests were carried out with the same coating solutions as in Example No.4. In these tests, the distance B between the slot outlet of anextrusion type head shown in FIG. 6 and the top of the intermediateblock 2 were changed to different values. The configuration of the endportion of the intermediate block was as shown in FIG. 4. The web usedas 15 μm in thickness and 500 mm in width, and the coating speed was 200m/min.

The coating rates of the first and second layers were set to 20 cc/m²and 10 cc/m², respectively. The width D of the slot was a constant 0.5mm. When the distance B was 3 mm or more, the extrusion-type head asshown in FIG. 6 was used, and when the distance B was 1 mm or less, anextrusion-type head as shown in FIG. 6B was used. The results of thetests are as indicated in Table 6 below:

                  TABLE 6                                                         ______________________________________                                        (mm)      Multi-layer surface                                                                           Evaluation                                          ______________________________________                                        30        No dual layer formed                                                                          X                                                   20        Dual layer formed, but                                                                        Δ                                                       some stripes found                                                  10        Dual layer formed, but                                                                        Δ                                                       some stripes found                                                  5         Dual layer formed, but                                                                        Δ ◯                                         some stripes found                                                  1         Satisfactory    ◯                                       0.05      Satisfactory    ◯                                       0.01      Satisfactory    ◯                                       ______________________________________                                         Legend:                                                                       ◯ Satisfactory                                                    X  Unsatisfactory                                                             Δ Average                                                          

What is claimed is:
 1. An extrusion-type coating head comprising:a rearblock defining a back edge; a front block defining a doctor edge; and anintermediate block disposed between said rear and front blocks, a firstslit being defined between said rear block and said intermediate blockfor accommodating a flow of a first coating solution, a second slitbeing defined between said front block and said intermediate block foraccommodating a flow of a second coating solution, a slot being formedbetween an end portion of said intermediate block and an outlet of saidhead, wherein said end portion is rounded with a radius of curvature ofno more than 1 mm.
 2. The extrusion-type coating head as claimed inclaim 1, wherein at least said end portion of said intermediate block isformed of a material having a Rockwell scale A hardness of at least 80.3. The extrusion-type coating head as claimed in claim 2, wherein saidmaterial having a Rockwell scale A hardness of at least 80 is selectedfrom the group consisting of carbon steel, nitride steel, cement steel,cemented carbide, and ceramics.
 4. The extrusion-type coating head asclaimed in claim 1, wherein said end portion has a flat end face havinga width of less than 1 mm.
 5. The extrusion-type coating head as claimedin claim 1, wherein said end portion has a flat end face having a widthof less than 0.5 mm.
 6. The extrusion-type coating head as claimed inclaim 1, wherein said end portion is formed by tapered sides that arefurther tapered near the tip to form a taper angle of at least 25°. 7.The extrusion-type coating head as claimed in claim 1, wherein said endportion is tapered to form a taper angle of at least 50°.
 8. Theextrusion-type coating head as claimed in claim 1, wherein a distancefrom a top of said end portion to a line joining tops of said doctor andback edges is in a range of 0.01 to 20 mm.
 9. The extrusion-type coatinghead as claimed in claim 1, wherein a distance from a top of said endportion to a line joining tops of said doctor and back edges is in arange of 0.01 to 5 mm.
 10. The extrusion-type coating head as claimed inclaim 1, wherein a distance from a top of said end portion to a linejoining tops of said doctor and back edges is in a range of 0.01 to 1mm.
 11. The extrusion-type coating head as claimed in claim 1, whereinwidths of said slits providing for the flow of each of said coatingsolutions at positions prior to said coating solutions meeting and awidth of said outlet providing for a multi-layer flow produced aftersaid coating solutions meet are each in a range of 0.03 mm to 3 mm. 12.The extrusion-type coating head as claimed in claim 11, wherein a ratioof flow rates between flows of each of said coating solutions is in arange of 1:1 to 5:1.
 13. An extrusion-type coating head comprising:arear block defining a back edge; a front block defining a doctor edge;and an intermediate block disposed between said rear and front blocks, afirst slit being defined between said rear block and said intermediateblock for accommodating a flow of a first coating solution, a secondslit being defined between said front block and said intermediate blockfor accommodating a flow of a second coating solution, a slot beingformed between an end portion of said intermediate block and an outletof said head, wherein said end portion is rounded with a radius ofcurvature of no more than 0.5 mm.